1. Field of the Invention
The present invention generally relates to a roller bearing and, specifically, to a needle roller bearing having a roller retainer of two-piece construction suited for mounting on, for example, an automotive transmission idler shaft or a big end of a connecting rod of a reciprocating engine.
2. Description of the Prior Art
The needle roller bearing of a type having a roller retainer is widely utilized in automotive vehicles and various kinds of industrial machinery because of its light-weight, compact structure. By way of example, in an automotive transmission comprising a synchro mechanism 50 shown in FIG. 4, the needle roller bearing is utilized as an idler bearing 53 through which a transmission gear 51 is mounted on a transmission shaft 52. According to the prior art, this type of needle roller bearing used as the idler bearing 53 is generally available in two types; one including a roller retainer of one-piece construction and the other including a roller retainer of split construction. By the reason which will now be described, the idler bearing 53 is generally preferred to employ the needle roller bearing of a type including the roller retainer of split construction to minimize a fretting phenomenon.
As is well known to those skilled in the art, when the automotive transmission is in a drive position with an automotive engine being loaded, the transmission gear 51 and the transmission shaft 52 rotate together therewith and, at this time, rotation of the idler bearing 53 relative to the transmission gear 51 and the transmission shaft 52 becomes zero. Under this condition, the needle rollers, generally identified by 54, undergo a minute vibratory motion at the same respective positions where they are positioned, rubbing respective outer and inner track surfaces of the inner and outer races 52 and 51. The consequence is that portions of the outer track surface of the inner race 52 aligned with the respective needle rollers 54 and portions of the inner track surface of the outer race 51 aligned also with the respective needle rollers 54 are worn to such an extent that an undulating pattern of indentations generally complemental to a pattern of the needle rollers, that is, a fretting marking, is left on the outer and inner track surfaces of the respective inner and outer races 52 and 51.
Where the roller retainer is of split construction having at least one crevice defined therein in a direction parallel to any one of the needle rollers, the crevice permits one portion of the roller retainer in a non-loaded region to undergo a relative motion in a direction circumferentially away from and close towards the other portion and, therefore, the possibility of occurrence of the fretting marking can be minimized as compared with the roller retainer of one-piece construction.
The split construction is known as available in two designs; one comprising a generally C-shaped roller retainer and the other comprising a pair of generally semicircular retainer segments of similar shape. The use of the roller retainer comprising the semicircular retainer segments can enhance a further minimization of the possible occurrence of the fretting marking.
Since the needle roller bearing of the type wherein the roller retainer is comprised of the retainer segments of similar shape permits a mounting of the roller retainer segments from a radial direction, this type of needle roller bearing is largely employed on a big end of a connecting rod. However, when the needle roller bearing comprising the retainer of two-piece construction is desired to be mounted in a manner similar to the idler bearing 53 which is, so far shown in FIG. 4, mounted on the shaft 52 by inserting it in a direction axially of the shaft 52, it is necessary that the two retainer segments have to be separately mounted. In addition, at the time the transmission gear 51 is to be mounted on the idler bearing 53 after the retainer 55 has been mounted on the shaft 52, the retainer 55 if composed of the two retainer segments may separate away from the shaft 52 and fall by gravity unless held in position by an assembly attendant worker or by the use of a tool, consequently constituting a cause of reduction in working efficiency.
Also, since the retainer segments are members separate from each other, the needle roller bearing comprising the retainer of two-piece construction requires an increased number of component parts, making it cumbersome and inconvenient to store and transport.
It is pointed out that, in the needle roller bearing comprising the roller retainer of one-piece construction, an attempt has been made to employ an increased number of needle rollers to minimize a contact pressure, or to employ an increased radial clearance between the shaft and the needle rollers, both in an attempt to further minimize the possible formation of the fretting marking. However, the increase in number of the needle rollers results in a reduction in space between each neighboring needle rollers which may in turn bring about a reduction in size of a pillar portion of the roller retainer existing in the space to cause a reduction in physical strength of the roller retainer. On the other hand, the increase in radial clearance over a certain limit will result in that the needle roller bearing in operation may generate a considerable noise and a vibration.